Not in a '39 I know, but I have connected the original master cylinder in my 50 truck to a remote booster mounted on the truck's bulkhead.

I have a CPP disc conversion on the front axle and drums at the rear. The brake setup works really well; I haven't needed to install any proportioning or residual pressure valves.

The booster I used was the model for an Austin Healey 3000 or Jaguar Mk 2; you'd probably find an American made unit is cheaper and easier to get hold of though.

Stuart.